The present invention is related to an improved method for preparing solid electrolytic capacitors. More specifically, the present invention is related to an improved method of forming a solid electrolyte capacitor and an improved capacitor formed thereby. Even more specifically, the present invention is related to an improved conformal cathode formed by improved polymerization method and improved metal plating methods particularly for fluted anodes.
The construction and manufacture of solid electrolyte capacitors is well documented. In the construction of a solid electrolytic capacitor a valve metal preferably serves as the anode. The anode body can be either a porous pellet, formed by pressing and sintering a high purity powder, or a foil which is etched to provide an increased anode surface area. An oxide of the valve metal is electrolytically formed to cover up to all of the surfaces of the anode and to serve as the dielectric of the capacitor. The solid cathode electrolyte is typically chosen from a very limited class of materials, to include manganese dioxide or electrically conductive organic materials such as 7,7,8,8 tetracyanoquinonedimethane (TCNQ) complex salt, or intrinsically conductive polymers, such as polyaniline, polypyrol, polythiophene and their derivatives. The solid cathode electrolyte is applied so that it covers all dielectric surfaces and is in direct intimate contact with the dielectric. In addition to the solid electrolyte, the cathodic layer of a solid electrolyte capacitor typically consists of several layers which are external to the anode body. In the case of surface mount constructions these layers typically include: a carbon layer; a cathode conductive layer which may be a layer containing a highly conductive metal, typically silver, bound in a polymer or resin matrix; and a conductive adhesive layer such as silver filled adhesive. The layers including the solid cathode electrolyte, conductive adhesive and layers there between are referred to collectively herein as the cathode layer which typically includes multiple interlayers designed to allow adhesion on one face to the dielectric and on the other face to the cathode lead. A highly conductive metal lead frame is often used as a cathode lead for negative termination. The various layers connect the solid electrolyte to the outside circuit and also serve to protect the dielectric from thermo-mechanical damage that may occur during subsequent processing, board mounting, or customer use.
Leading edge designs for portable electronic devices require capacitors which deliver volumetric efficiency, high reliability, low cost, reduced propensity for ignition, and low Equivalent Series Resistance (ESR). Tantalum capacitors employing a conductive polymer cathode are uniquely capable of providing all of these critical characteristics. Tantalum capacitors have a long track record for unsurpassed volumetric efficiency and reliability. The introduction of conductive polymer cathodes enabled dramatic reductions in ESR and has greatly increased resistance to ignition relative to MnO2. The lower ESR and enhanced cap retention at high frequency of the conductive polymer construction enables circuit designers to reduce the number of capacitors required to achieve design solutions resulting in reduced cost.
U.S. Pat. No. 7,154,742, which is incorporated by reference, describes the use of capacitive elements with very narrow flutes to improve capacitance recovery. As illustrated in FIGS. 5-8 of the '742 patent the use of flutes is considered in the art to be suitable for a limited range of flute size. U.S. Pat. Nos. 7,342,775 and 7,116,548, which are incorporated herein by reference, improve on the fluted capacitor by incorporating multiple lead wires and a conductive polymer cathode system yet the flute size is still somewhat limited and the full potential expected with the flutes is still not realized. While not limited to any theory it has now been realized that the limitation of flute sizes results from the inability of the cathode layers to conform to the flute dimensions. As illustrated in FIG. 2 of U.S. Pat. No. 7,342,775, the cathode layers tend to pool in the flute and therefore the thickness of cathode is inconsistent thereby limiting the advantages provided by the flutes. This problem, now realized, was considered unavoidable since the methods available for forming the cathode layer did not lend themselves to conformal coatings and the full advantages available with fluted anodes was considered to have been achieved.
The present invention provides an improved capacitor wherein additional advantages, beyond those thought achievable, from a fluted anode can be achieved.